Exercise testing has been widely applied for the evaluation of patients with coronary artery disease. The principles underlying its use for this indication make it a useful adjunctive technique, when combined with ambulatory monitoring, to diagnose arrhythmias and monitor antiarrhythmic drug therapy. During exercise, there is a withdrawal of vagal tone and a marked increase in circulating catecholamines and sympathetic inputs to the heart. These changes may directly cause arrhythmias (e.g., catecholamines can enhance automaticity and delayed afterpotentials and can shorten myocardial conduction time and refractory periods). However, they also augment myocardial oxygen demands by increasing myocardial inotropy, heart rate and blood pressure. Such changes may cause ischemia in patients with heart disease, which is a powerful stimulus for arrhythmia, or lead to dysfunction in left ventricular contraction and increased myocardial wall stress, factors that also may precipitate arrhythmia. In approximately 10% of patients with a history of serious arrhythmia, exercise represents the only means for exposing arrhythmia. Importantly, this technique is useful for evaluating the effect of antiarrhythmic drugs. These agents work by reducing membrane automaticity, slowing impulse conduction through the myocardium and prolonging membrane refractoriness. In contrast, catecholamines, which are secreted in response to exercise, have the opposite effect. Thus, exercise may negate the important effects of the antiarrhythmic drugs. Additionally, exercise testing may expose potentially serious toxic drug reactions that may not be obvious at rest. These include conduction abnormalities, negative inotropic effects, congestive heart failure and aggravation of arrhythmia. Although the presence and frequency of arrhythmia with exercise is highly variable in patients with benign arrhythmia, results are more consistent in patients with a history of serious arrhythmia. If arrhythmia is reproducibly provoked with exercise, this technique can be used to judge drug effect. Thus, exercise testing is an important, reliable and helpful technique for exposing arrhythmia, evaluating drug efficacy and identifying potentially serious toxic drug effects.